Negotiators for Iran and six world powers on Tuesday focused for the first time on the concrete shape of a deal meant to result in long-term curbs on Tehran’s nuclear program.

With less than three months to go before an informal July deadline, Iranian Foreign Minister Mohamad Javad Zarif said the talks Tuesday and Wednesday in Vienna are meant to start work on drafting the text of an agreement. Initial rounds in February and March dealt in generalities and work on the text “is the difficult part,” he said.

“Probably at the beginning of preparing the draft, the differences will be lots,” he told Iran’s state-run television.

The world powers are offering to remove sanctions that have crippled Iran’s economy if a deal can be struck.

The scope of Iran’s uranium enrichment program is the toughest issue. Iran argues it needs robust enrichment capacities in order to make low-enriched reactor fuel. The U.S., Britain, France and Germany want significant cuts, to limit the potential for the program to be used for making high-enriched material for nuclear arms. Russia and China are somewhere in the middle.

The six also want to eliminate potential proliferation dangers from an enrichment site at Fordo, south of Tehran, that is built far underground to withstand air strikes; and a nearly built nuclear reactor at Arak, in northwestern Iran, that would produce substantial amounts of plutonium unless it is re-engineered to new specifications.

Like enriched uranium, plutonium can be used for the core of a nuclear weapon.

Zarif said the two sides would “review all the topics” Tuesday and Wednesday but cautioned of tough work ahead.

“There are some issues that have been partially touched upon but have not been broken down yet or need to be reconsidered,” he said.

A first-step deal, in effect since January, has curbed some Iranian nuclear activities in return for limited sanctions relief as the two sides work toward a final agreement.